Steep sampling

The investigation team at an earth-fill dam negotiates a tricky incline
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Ever since its introduction to the UK ground investigation sector, slope-climbing plant and equipment has been changing the way boreholes are being constructed on projects where access is limited by steep inclines. Scaffold platforms and crane lifts are now being replaced by specialist equipment that is capable of tracking up and down slopes, providing a safe working platform to undertake the necessary sampling and drilling works.

Many of the UK's earth-fill dams were built in the late 19th or early 20th century, and while well-constructed with the tools and technology available at the time, a decline in effectiveness and the requirement for maintenance are inevitable. Consequently, Geotechnical Engineering Limited's (GEL's) P60 slope-climbing equipment has been operating on an earth-fill reservoir embankment in the north of England for design and construct specialist Mott MacDonald Bentley.

Anchored in place

During pre-site visits, it became apparent that access was restricted at the crest by a narrow track and berm, and so the only option to get plant to the proposed positions was from the toe of the slope. This access route was challenging in itself, as all plant and equipment would need to negotiate a deep, steep-sided bowl-shaped depression that lay between the site access point and the base of the slope.

To this end, a strategy was determined by GEL's health and safety manager whereby a series of Vulcan AS-90 earth anchors (supplied by Anchor Systems UK) would be installed, and load-tested to meet the design criteria determined by GEL's technical director. The anchors would be used in sequence to allow the machines to be tethered securely during both the access to the toe of the embankment and subsequently when working on the slope itself. While traversing the slopes, rigs would not release from one anchor set unless safely tethered to the next.

The anchors for the downstream slope were installed in two phases given that very little space was available at the crest of the slope and to avoid the loaded anchors damaging the clay embankment core. The first phase consisted of two anchors from which the crew would be tethered when installing the main anchor array a few metres down the slope. Each operative was equipped with a safety line attached to a harness and fall-arrest system. Where necessary, steps or terraces were also dug into the slope to provide a level area on which to stand.

Once installed, the anchors were proof-tested to a load equal to the serviceability limit set out in the anchor design document, all in accordance with BS EN 1997-1:2004+A1 and BS EN 1537:2013. A set of webbing slings was subsequently attached to each anchor point when it was necessary to connect up to the P60 rig and tender winchlines.

The rig and tender both provided a safe elevated work platform (EWP) on which the drill crew could operate to construct the borehole. EWPs are all inspected and tested every six months to comply with LOLER (Lifting Operations Lifting Equipment Regulations) and are equipped with hand rails and toe boards to protect site staff of the risks of working at height. The machines are tracked by radio remote control from an upslope position of safety.

For one of the boreholes, an additional set of anchors was installed to allow a second tender to service the drilling operations. The borehole was located almost directly above the dam spillway, and so the original tender, which sits to the right of the rig to extend the working area, could not be moved from position without fouling the anchor lines. The extra slope-climbing carrier therefore overcame this challenge and provided a means of moving equipment and consumables up and down the slope to the rig. For daily pedestrian access on and off the rigs, the client provided proprietary Haki type stairs to allow the crew to safely navigate the steep embankment slope.

Investigation methods

Overseen by GEL engineering geologists, the investigation itself comprised four dynamic-sampled and rotary-cored boreholes to a specified depth of 40.0m below ground level, with standard penetration testing throughout, variable head testing in the initial two boreholes and packer permeability testing in the final two locations. The boreholes were advanced by heavy duty dynamic sampling through the shallow soils and once into stiff over-consolidated clays over bedrock, by a conventional T6-116 triple-tube system using a water flush to produce 90mm-diameter core. At depth, the bedrock comprised millstone grit formation. Where the ground conditions dictated, a reducing string of casing was installed, and the holes were continued using a T6-H barrel.

The investigation was undertaken by a fully trained and briefed field crew, who successfully navigated and overcame the challenging terrain to access the positions and work safely on the embankment slope. All operations were completed under the close and direct supervision of Mott MacDonald Bentley, which ensured the works complied fully with the detailed, written safe systems of work. The boreholes were all drilled to the satisfaction of the client with consistently good sample and core recovery throughout.

Demands for this type of specialist equipment is such that GEL is currently investing in the next generation of its multi-purpose P60 slop-climbing sampling and coring rig.

This article was written by Tom Worsley, senior contract manager for Geotechnical Engineering Limited, with the permission of Mott MacDonald Bentley


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